Rotation control devices of this type operate in accordance with the reluctance principle, in that a torque acting against a restoring spring is generated via the angle of rotation of a rotating armature by a change of the magnetic resistance in the working air gap between the pole pieces of the rotating armature and the magnetic poles of the magnet housing. In this case the magnetic resistance can be affected by the cross section of the active magnetic field, the field length and the relative permeability.
In a known rotation control element of this type (German Patent Disclosure DE 35 22 993 A1; U.S. Pat. No. 4,724,349 and U.S. Pat. No. 4,494,517), the gradient of surface overlapping of the magnetic pole and pole piece is utilized via the angle of rotation of the rotating armature. For this purpose the pole pieces are embodied in a wedge shape, viewed crosswise to the axis of rotation of the rotating armature, where the flanks, located opposite each other in the axial direction and running towards each other, extend straight or in a curve in order to realize a required flow-through characteristic. In the case of small surface overlapping, changes in the relative permeability of the sinter metal of the pole pieces are superimposed, where the partial areas extend as far as the saturation induction. This saturation acts in the way of an enlargement of the air gap. The amount of the air gap enlargement cannot be practically determined and in its use means an increase in the tolerance of the drive torque and thus of the characteristic line of the rotation control element.